1. Field of the Invention
The present invention relates to an electrostatic image developing toner for use in electrophotographic image formation using copiers, electrostatic printing, printers, facsimiles, electrostatic recording, etc., and a developer using the toner, an image forming apparatus, an image forming method and a process cartridge each using the toner.
2. Description of the Related Art
Conventionally, in electrophotographic image forming apparatuses, electrostatic recording apparatuses, etc., electric or magnetic latent images are formed into visible images with toners. For example, in an electrophotographic image formation process operation, an electrostatic image (a latent image) is formed on a photoconductor and then developed thereon using a toner to form a toner image. The toner image is transferred, generally, onto a recording medium such as paper, and then fixed on the recording medium by heating or the like.
Toners for use in developing of electrostatic images are generally colored particles containing a colorant, a charge controlling agent and the like in a binder resin. The production methods of such toners are broadly classified into pulverization methods and suspension polymerization methods.
In the pulverization method, a colorant, a charge controlling agent, an offset preventing agent and the like are uniformly dispersed in a thermoplastic resin by melt-mixing to obtain a toner composition. Then, the toner composition thus obtained is pulverized and subsequently classified, thereby producing a toner. According to the pulverization method, it is possible to produce a toner excellent in physical properties to some extent, however, the selection of materials is limited. For example, a toner composition obtained by melt-mixing needs to be able to be pulverized and classified by an economically usable device. To respond to the demand, the toner composition obtained by melt-mixing must be made brittle. When such a toner composition is pulverized, particles having a broad particle size distribution are likely to be formed. At this time, in an attempt to obtain a copied image having excellent resolution and gradation, for example, fine particles each having a particle diameter of 5 μm or less and coarse particles each having a diameter of 20 μm or more must be eliminated by classifying the toner particles, thereby causing substantially low toner yield. In addition, in the pulverization method, it is hard to uniformly disperse a colorant and a charge controlling agent and the like in a thermoplastic resin. A dispersion solution in which these components are insufficiently dispersed adversely affects flowability developing property and durability of the resulting toner, image quality, and the like.
In order to overcome the problems in the pulverization method, Japanese Patent Application Laid-Open (JP-A) Nos. 09-319144 and 2002-284881 propose toner production methods through a dissolution suspension method using a dissolved resin (hereinbelow, otherwise referred to as “resin-dissolution-suspension method”). This dissolved resin is preliminarily synthesized by a polymerization reaction and later dissolved in a liquid to prepare a resin solution, which is dispersed in an aqueous medium in the presence of a surfactant or a dispersing auxiliary (a water-soluble resin, etc.) and a dispersion stabilizer (inorganic fine particles, resin fine particles, etc.), followed by heating, or reduction of pressure or the like, to remove solvents therein. The resin-dissolution-suspension method enables production of a toner having a uniform particle size, without the need for classification of the resin dispersion liquid.
In addition, in electrophotographic image forming apparatuses, toners are required to have good releasability (hereinbelow, otherwise referred to as “offset resistance”) from heating members during a fixing step based on a contact-heating process in which a heating member such as a heating roller is used to fix an image. An improvement of the offset resistance of a toner prepared by the resin-dissolution-suspension method is achieved by using a modified polyester resin (see Japanese Patent (JP-B) No. 3640918).
Meanwhile, most of binder resins accounting for 70% or more of the total amounts of toner components are derived from petroleum resources. There are concerns about exhaustion of petroleum resources and concerns that a large amount of petroleum resources is consumed and a large quantity of carbon dioxide is released into the atmosphere, leading to global-warming. Then, when resins derived from plants taking in carbon dioxide in the atmosphere to grow are used as binder resins, carbon dioxide generated in use of the toners only circulates in the environments, and the use of plant-derived resins may make it possible to simultaneously solve both problems of global-warming and the exhaustion of petroleum resources. A variety of toners using such plant-derived resins as binder resins have been proposed. For example, Japanese Patent (JP-B) No. 2909873 proposes to use a polylactic acid as a binder resin. However, when a polylactic acid is directly used as a binder resin according to the proposal, the concentration of ester linkage of the binder resin is higher than that of a polyester resin, and thus, the effect as a thermoplastic resin becomes weak in fixing step of toner image. Moreover, the toner becomes very hard, lacking in pulverizability, and resulting in degradation of productivity. Furthermore, the toner becomes very hard, lacking in pulverizability, and resulting in degradation of productivity.
Japanese Patent Application Laid-Open (JP-A) No. 09-274335 proposes an electrostatic image developing toner, which contains a polyester resin obtained by dehydration polycondensation of a composition containing a lactic acid, and a trifunctional or higher-functional oxycarboxylic acid, and a colorant. However, in this proposal, since the polyester resin is formed by a dehydration polycondensation reaction between a hydroxyl group of the lactic acid and a carboxyl group of the oxycarboxylic acid, the molecular weight is increased, thereby causing impairment of the sharp-melt property and low-temperature fixability.
In order to improve thermal properties of toner, Japanese Patent Application Laid-Open (JP-A) No. 2001-166537 discloses an electrophotographic toner containing a polylactic acid-based biodegradable resin and a terpene-phenol copolymer, which however, cannot satisfy both the low-temperature fixability and the hot-offset property simultaneously.
Since the toners relating to these proposals are obtainable by a pulverization method, it involves problems of toner loss caused by classification, and toner waste accompanied therewith. In addition, because the energy quantity required for performing the pulverization method is relatively large, it is necessary to further reduce environmental load.
Polylactic acids, which are generally used and easily available as resins derived from plants, are synthesized by dehydration condensation of a lactic acid, as described in Japanese Patent Application Laid-Open (JP-A) Nos. 07-33861 and 59-96123, or by ring-opening polymerization of a cyclic lactide of lactic acid. For this reason, when a toner is produced using a polylactic acid, the dissolution suspension method using a dissolved resin, as disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 09-319144, 2002-284881 and Japanese Patent (JP-B) No. 3640918, can be used. However, since a polylactic acid having only L form or D form has high crystallinity, the solubility in organic solvents is extremely low, and thus it is difficult to use dissolution suspension method using a dissolved resin. To overcome the problem, Japanese Patent Application Laid-Open (JP-A) No. 2008-262179 discloses that the solubility of lactic acid in organic solvents can be improved by mixing L form of a polylactic acid and D form of a polylactic acid to decrease the crystallinity.
On the other hand, since it is difficult to control the molecular weights of polylactic acids, and ester linkages are present via only carbon atoms, it is difficult to impart necessary physical properties to toner by using polylactic acid along. In contrast, as used in conventional methods, it can be considered to provide necessary physical properties and thermal properties to toner by using a mixture of a polylactic acid and other resin or resins. However, polylactic acids are extremely poor in solubility and dispersibility in polyester resins and styrene-acryl copolymers which are generally used for toner, and thus it is very difficult to produce a toner in such a manner.
Furthermore, since the rate of crystallization of polylactic acids is rather slow, a toner produced by dissolution suspension method using a dissolved resin is difficult to control the crystallized state of polylactic acid, and in a toner produced by the method, a polylactic acid having high-crystallinity and a polylactic acid having low-crystallinity are present in a mixed manner. Therefore, portions having the high-crystalline polylactic acid are grown into crystals over time, causing changes in charged amount and image density of the resulting toner as time goes by.
Further, polylactic acid has a number of polar groups per unit structure, and thus when a toner is produced using a polylactic acid whose crystallinity has been reduced, the resulting toner is largely influenced by humidity as compared with using a polylactic acid having high crystallinity. Therefore, it is difficult to control the charged amount of toner. Particularly, it is difficult to reduce variations in charged amount under low-temperature and low-humidity conditions, and high-temperature and high-humidity conditions. For this reason, in use of polylactic acids, there are such drawbacks that the charged amount and the image density are unstable.
Accordingly, a toner which are superior in image density, haze degree, fixability, and heat-resistant storage stability, causes less changes in fixability with a lapse of time and contains a polylactic acid, and the related techniques have not yet been obtained, and further improvements and developments are still desired.